This op creates a new composite constant with one element replaced. So,
we reconstruct the `SPIRConstant` for the composite constant, but with
one of the IDs replaced. Constant initializer lists are memoized for
when the result of a `CompositeInsert` is used in another
`CompositeInsert`.
(I wanted to add a test case for GLSL as well, but for two things:
1. `glslang` in Vulkan mode chokes on the first constant array,
insisting that its initializer needs to be a constant. [Bug in
glslang?]
2. The declarations for the buffers used by the shader aren't emitted,
regardless of whether Vulkan mode is enabled.)
Fixes five tests under
`dEQP-VK.spirv_assembly.instruction.*.opspecconstantop.vector_related`.
